1. Field of the Invention
This invention relates to rotary body drive devices, and, more particularly, to such drive device which effectively performs rotational driving of a record bearing medium.
2. Description of the Prior Art
In a rotary body drive device, such as a device for rotational driving of, for example a magnetic disk in, for example, an electronic still camera, the driving of a motor has been controlled in such a manner that its phase is brought into coincidence with a reference signal for controlling the timing of operations of the camera as a whole. It has, therefore, taken, a long time to control the phase of the motor. This has led to degradation in the response characteristics of the camera.
Also, as the method of phase control, it may be considered that the phase of a pulse of a frequency generator (FG) cooperative with the motor is synchronized in delay of .pi. with that of the reference signal. In this case, however, the dynamic range of phase control is limited to comform with a range of delayed phases of the FG pulse by 0 to 2.pi. from the reference signal.
Yet, by a sharp load change on the motor or the like, the phase difference will probably exceed the aforesaid controllable range. For example, if, as shown in FIG. 1, the phase difference between the reference signal and the FG pulse sampled for an (n-1)st cycle of control has been about (15/8).pi. and the phase of the FG pulse is delayed by more than (1/8).pi. from the reference signal at the next sampling time, or until the n-th cycle, the phase delay of the FG pulse from the reference signal amounts to more than 2.pi.. By such a phase control method as described above, however, in this case, the phase discrepancy will be determined to become smaller. Thus, the prior art method had a drawback that proper control becomes at times impossible.